The production process of thermo-expandable microcapsules wherein thermoplastic polymer encapsulates a volatile expansion agent which gasifies at a temperature lower than the softening point of the polymer has already been disclosed, for example, in Japanese Patent Publication Sho 42-26524.
Japanese Patent Laid-Open Sho 60-19033 (equivalent to U.S. Pat. No. 4,582,756) discloses a process wherein the surface of micro-balloons is covered with thermosetting resin to produce solvent-resistant micro-balloons that can be added into hot molten resin.
Japanese Patent Publication Hei 5-15499 and Japanese Patent Laid-Open Hei 5-285376 (equivalent to U.S. Pat. No. 5,536,756) disclose a process wherein a monomer mixture comprising 80 percent or more of nitrile monomer having polymerizable unsaturated bonds form the shell of microcapsules that have superior heat and solvent resistance.
The microcapsules produced in the processes are blended in synthetic resins to impart matte and bulky effect to construction materials (materials for ceiling, wall, floor and the like) or coating materials for bags.
Japanese Patent Laid-Open Hei 7-304968 (equivalent to U.S. Pat. No. 5,726,222) discloses the microcapsules comprising a shell of which main component is (B) a thermoplastic resin soluble in a thermosetting resin under heating and (A) a curing agent of the thermosetting resin being encapsulated in the shell. It discloses that the microcapsules contribute to the production of a thermosetting resin composition or prepreg stable under storage at normal temperature and fiber-reinforced composite having uniform hardness and superior mechanical property. And polyether imide and polyimide are exemplified as the thermoplastic resin, (B), and maleimide resin is exemplified as the thermosetting resin.
Microcapsules need to be heated for expansion and their color changes depending on the heating temperature, remarkably at 200.degree. C. or more. None of the thermo-expandable microcapsules developed in the past as mentioned above was durable against the color change under heating at 200.degree. C. or more and retained matte effect.
The object of the present invention is to provide thermo-expandable microcapsules which can be used to produce synthetic resin foam layer having matte and bulky surface and durable against thermal coloration especially under heating at 200.degree. C. or more and to provide resin foam and foamed resin composite produced with the same to solve the above problem.